1. Field of the Invention
The present invention relates to global location satellite systems and in particular to a high sensitivity automatic frequency control for GLONASS or GPS.
2. Related Art
GPS (global positioning system) and GLONASS (global navigation satellite system) are radio-based satellite systems in operation today. To provide global coverage, GPS uses between 24-32 satellites. Assuming the minimum number of 24 satellites, 4 satellites are deployed in each of six orbits. The six orbital planes' ascending nodes are separated by 60 degrees. In this configuration, a minimum of six satellites should be in view from any given point at any time. To provide global coverage, GLONASS includes 24 satellites, wherein 21 satellites can be used for transmitting signals and 3 satellites can be used as spares. The 24 satellites are deployed in three orbits, each orbit having 8 satellites. The three orbital planes' ascending nodes are separated by 120 degrees. In this configuration, a minimum of five satellites should be in view from any given point at any time.
Both GPS and GLONASS broadcast two signals: a coarse acquisition (C/A code) signal and a precision (P code) signal. In general, global position devices, called receivers herein, lock onto the C/A transmission and not the P transmission. The P transmission is much longer than the C/A transmission and therefore is impractical to directly lock onto. Once a lock is established via C/A transmission, the C/A transmission itself can provide a quick P lock.
The C/A codes for GPS and GLONASS, which can be generated as a modulo-2 sum of two maximum length shift register sequences, are selected for good cross-correlation properties. Each GPS satellite transmits its own unique C/A code, which has an identifiable pseudo-random noise code number (PRN#). In contrast, each GLONASS satellite transmits the same C/A code, and is identified by its channel number (CHN#).
The C/A code includes navigation data, which provides information about the exact location of the satellite, the offset and drift of its on-board atomic clock, and information about other satellites in the system. In GPS, the C/A format for the navigation data includes words, frames, and subframes. The words are 30 bits long; ten words form one subframe; and five subframes form one frame. In GPS, the C/A code is 1023 bits long, is transmitted at 1.023 Mbps, and therefore has a repetition period of 1 ms. In GLONASS, the C/A format is strings, wherein each string includes 1.7 sec of navigation data and 0.3 sec of a time mark sequence. Notably, the C/A code in GLONASS is 511 bits long, is transmitted at 511 kbps, and therefore has the same code repetition period (i.e. 1 ms) as GPS.
Automatic frequency control (AFC) is used to automatically keep a GPS/GLONASS receiver tuned to the frequency of an incoming GPS/GLONASS signal. This tuning is desirable because of a natural drift of the center bandpass frequency of the receiver, which is caused by satellite and user movement, and thermal or mechanical drift in receiver component values.
With the advent of GLONASS satellites now being available to provide position information, it is desirable to have a system that includes the capability of using GPS and/or GLONASS signals for position determination. Therefore, a need arises for a method for providing AFC for GPS, GLONASS, and hybrid GPS/GLONASS receivers.